1. Field of the Invention
The present invention relates to the field of tunable lasers. More specifically, it relates to a system and method for improving wavelength stabilization while also improving mode stabilization of DBR lasers.
2. Description of the Related Art
Optical fiber communications systems provide for low loss and very high information carrying capacity. In practice, the bandwidth of optical fiber may be utilized by transmitting many distinct channels simultaneously using different carrier wavelengths. The associated technology is called wavelength division multiplexing (WDM).
The wavelength bandwidth that any individual channel occupies depends on a number of factors, including the impressed information bandwidth, and margins to accommodate carrier frequency drift, carrier frequency uncertainty, possible inter-channel cross-talk due to non-ideal filters.
To maximize the number of channels, lasers with stable and precise wavelength control are required to provide narrowly spaced, multiple wavelengths. One such laser is the distributed Bragg Reflector (DBR) laser. The DBR laser is a device whose wavelength can be tuned electrically as well as thermally over a range of 8 nm to 22 nm.
Under normal operating conditions of a DBR laser, a thermal electric cooler (TEC) is used to xe2x80x9cfine tunexe2x80x9d the output wavelength of the laser through a temperature compensating feedback loop. The temperature compensating feedback loop is activated once the current to the tuning section of the laser has been applied to place the laser wavelength close to its desired operating point.
In this situation, the tuning current can be said to be in a xe2x80x9cstaticxe2x80x9d or xe2x80x9cidlexe2x80x9d state, held to a constant value while the temperature compensation feedback loop maintains a stable wavelength via an ever present servo about this desired operating point. Under these conditions, the control system will employ some constant numeric gain in response to this temperature loop control signal which is ideal to maintain wavelength stability with as small a servo as possible about the desired operating point.
However, due to the nature of the DBR laser, a periodic adjustment (up or down) of the current to the tuning section of the laser is required as a means to determine an ideal operating point with respect to mode stabilization of the laser. Such a ramping of the current to the tuning section has the immediate undesirable effect of tending to shift the wavelength of the laser. That is, the tuning current that is optimal for mode stabilization (or some other operating condition such as side mode suppression), may not correspond to the tuning current required to maintain a static wavelength. Therefore, temperature compensation is required to retain the desired wavelength.
Ramping the current to the tuning section with the temperature compensation loop engaged does reduce this tendency to shift in wavelength by thermally adjusting the laser""s wavelength via the temperature feedback loop in response to the delta""s induced by altering the tuning current. However, depending on the thermal response time of the system, the mitigating effect of the temperature compensation feedback loop (i.e., to reduce the shift in wavelength) is not always ideal. Often, the wavelength has drifted farther than desired during the ramping of the tuning section current because the system cannot adequately respond via temperature compensation in a time frame consistent with the required rate of change of the tuning current ramp.
A principle reason for the above described problem is the fact that the numeric gain applied to the temperature loop, while presumably ideal to minimize the servo about the control point while the tuning current is in a xe2x80x9cstaticxe2x80x9d state, is often not responsive enough to ideally compensate for wavelength drift when a tuning current ramp is in progress. Therefore, it is desirable to have a temperature compensation feedback loop in which the numeric gain can be dynamically increased (or suitably altered) as required to improve the responsiveness of the temperature loop when a tuning current ramp is in progress.
The present invention provides a system and method for dynamically increasing the numeric gain applied to the temperature loop in a DBR laser control system in response to the anticipated wavelength change that occurs as a tuning current ramp is in progress. In accordance with a preferred embodiment of the invention, the feedback control signal to the laser""s TEC is overcompensated for, and as a result, the system is forced to respond much faster, thermally, to the anticipated wavelength shift, thus providing improved real time wavelength stabilization by the control system. The implementation of the increased numeric gain can be carried out through software and/or firmware.